Small-angle X-ray scattering (SAXRS) is a well-known method for surface layer characterization. It is described, for example, by Parrill et al., in “GISAXS—Glancing Incidence Small Angle X-ray Scattering,” Journal de Physique IV 3 (December, 1993), pages 411–417, which is incorporated herein by reference. In this method, an incident X-ray beam is totally externally reflected from a surface. The evanescent wave within the surface region is scattered by microscopic structures within the region. Measurement of the scattered evanescent wave can provide information about these structures. For example, Parrill et al. describe the use of this technique for determining size information regarding islands associated with film growth on the surface.
SAXRS can be used in this manner to determine characteristics of pores in a surface layer of a low-k dielectric material formed on a silicon wafer. Nano-porous silicates and polymers are considered to be attractive materials for use in microelectronic devices with sub-0.25 μm technology, but non-destructive characterization of pore size and density has so far proved to be a difficult task. The use of diffuse X-ray reflectivity in characterizing porous low-k materials is described, for example, by Wormington in “Characterization of Pore Size Distribution in Low k Dielectrics Using X-ray Reflectivity,” presented at the Sematech Gate Stack Engineering Workshop (Austin, Tex., May 2, 2002), which is incorporated herein by reference. A similar method is described by Ito in “X-ray Scattering Method for Determining Pore-Size Distribution in Low-k Thin Films,” presented at the International Sematech Ultra Low-k Workshop (San Francisco, Calif., Jun. 6–7, 2002), which is also incorporated herein by reference.
Known techniques for SAXRS use a monochromatic, collimated beam of X-rays to irradiate the surface of the sample. X-ray collimators and monochromators that are known in the art tend to be very inefficient. Therefore, the flux of the monochromatic, collimated X-ray beam on the surface of the sample is generally very weak, leading to poor signal/noise ratio and low throughput in the scattering measurements. A number of attempts have been made to increase the useful X-ray flux for scattering measurements. For example, Iwasaki describes an X-ray optical device and multilayer mirror for use in a small angle scattering system in U.S. Patent Application Publication U.S. 2001/0028699 A1, whose disclosure is incorporated herein by reference. The multilayer mirror has elliptical reflection faces, which have two focal points. Thus, an X-ray beam from a source at one of the focal points is focused to a spot at the other focal point in a manner that is said to provide high precision in small-angle scattering measurements.